TRENES

Pregnancy Calculator

See how many weeks pregnant you are, your trimester and your estimated due date.

Written by Kübra Yıldız · Reviewed by Furkan Tur · Updated

Leave at 28 if unsure; the same correction as the due date calculator is applied

Your result will appear here

Fill in the fields and press Calculate.

“How many weeks pregnant am I?” has a slightly surprising answer: pregnancy is counted from the first day of your last menstrual period (LMP), which is about two weeks *before* you actually conceived. That’s the convention every doctor, scan and pregnancy book uses, so it’s the one this tool uses too.

Enter the first day of your last period and today’s date. You’ll get your gestational age in weeks and days, your current trimester, and your estimated due date.

Pregnancy Calculator: step-by-step worked example

Example inputs
First day of last period
May 15, 2026
Today’s date
October 12, 2026
Cycle length (days)
28
  1. 1

    Days since the last period

    Days = Today − Last period

    October 12, 2026 − May 15, 2026 = 150 days

  2. 2

    Cycle-length correction

    Gestational days = Days − (Cycle − 28) · no correction for a 28-day cycle

    max(0, 150 − (28 − 28)) = 150 days

  3. 3

    Week of pregnancy

    Weeks = ⌊Gestational days ÷ 7⌋

    ⌊150 ÷ 7⌋ = 21 weeks

  4. 4

    Extra days

    Extra = Gestational days mod 7

    150 mod 7 = 3 days

  5. 5

    Trimester

    < week 14: 1 · 14–27: 2 · 28+: 3

    = 2

  6. 6

    Due date and time left

    Due = Last period + 280 days + (Cycle − 28) · Left = Due − Today

    May 15, 2026 + 280 = February 19, 2027 · 130 days

Result · Pregnancy week21 weeks
Where you are in the pregnancy
  1. Last period
    May 15, 2026
  2. 2nd trimester
    August 21, 2026
  3. Today
    October 12, 2026
  4. 3rd trimester
    November 27, 2026
  5. Due date
    February 19, 2027

Good to know: Pregnancy weeks are counted from the first day of the last period, not from conception (gestational age), so the first two weeks are technically before pregnancy. By default the estimate assumes a regular 28-day cycle (Naegele’s rule); if you enter your cycle length, the week and due date shift by (cycle − 28) days, matching the due date calculator. If a first-trimester scan gives a different date, go with your doctor’s.

Pregnancy Calculator — how it works

Gestational age, in weeks and days

Gestational age is simply the time elapsed since your LMP, expressed as full weeks plus extra days — for example “14 weeks + 3 days”. The extra days matter because a single week spans several developmental changes.

ValueCalculation
Days pregnantToday − first day of last period
Weeks + daysDays ÷ 7 (whole weeks, remainder days)
Estimated due dateLMP + 280 days
Days remaining280 − days pregnant

Why it’s about 2 weeks “ahead” of conception

At the start of a cycle you aren’t pregnant yet — ovulation and conception happen around two weeks in. But dating from the LMP is far more reliable than guessing the conception date, so by convention week 1 begins before conception. This is why a “40-week” pregnancy is really about 38 weeks of fetal development.

Trimesters

TrimesterWeeksNotes
First0 – 13Organs form; early scans and screening
Second14 – 27Anomaly scan; often the most comfortable stretch
Third28 – birthGrowth and preparation for delivery

Accuracy

LMP dating assumes a regular cycle and a known period date. A first-trimester ultrasound measures the baby directly and is more precise; if it disagrees with the LMP date, clinicians re-date the pregnancy from the scan.

Worked example

If your last period began on 1 January 2026 and today is 9 April 2026, that’s 98 days — exactly 14 weeks + 0 days, placing you at the start of the second trimester. Your estimated due date is 1 January + 280 days = 8 October 2026, with about 182 days remaining. Because dating runs from the LMP, conception was around mid-January, so fetal development is roughly two weeks behind the “14 weeks pregnant” figure.

Estimated due date and pregnancy week as of 12 October 2026, by last period date

Assumes a regular 28-day cycle (Naegele’s rule: first day of last period + 280 days). An ultrasound may adjust the date.

Last periodDue dateWeek todayTrimester
1 January 20268 October 202640 weeks 4 days3
15 January 202622 October 202638 weeks 4 days3
1 February 20268 November 202636 weeks 1 day3
15 February 202622 November 202634 weeks 1 day3
1 March 20266 December 202632 weeks 1 day3
15 March 202620 December 202630 weeks 1 day3
1 April 20266 January 202727 weeks 5 days2
15 April 202620 January 202725 weeks 5 days2
1 May 20265 February 202723 weeks 3 days2
15 May 202619 February 202721 weeks 3 days2
1 June 20268 March 202719 weeks 0 days2
15 June 202622 March 202717 weeks 0 days2
1 July 20267 April 202714 weeks 5 days2
15 July 202621 April 202712 weeks 5 days1
1 August 20268 May 202710 weeks 2 days1
15 August 202622 May 20278 weeks 2 days1
1 September 20268 June 20275 weeks 6 days1
15 September 202622 June 20273 weeks 6 days1

Frequently asked questions

How many weeks pregnant am I?+

Count the days from the first day of your last period to today, then divide by 7 for whole weeks plus leftover days — e.g. 98 days is 14 weeks + 0 days. This tool does it for you and also shows your trimester and due date.

Why is pregnancy dated from my last period and not conception?+

Because the LMP is a date you can identify reliably, while the exact day of conception usually isn’t known. Conception happens about two weeks after the LMP, so gestational age runs roughly two weeks ahead of the actual age of the fetus.

What trimester am I in?+

The first trimester is weeks 0–13, the second is 14–27, and the third is 28 until birth — all counted from your last period. The calculator shows your current trimester from the dates you enter.

How is the due date worked out?+

By adding 280 days (40 weeks) to the first day of your last period. It’s an estimate: a first-trimester ultrasound is more accurate and is used to re-date the pregnancy if it differs from the LMP calculation.

Is this a substitute for medical care?+

No. It’s an informational estimate based on the dates you enter, not medical advice. Your dates and due date should be confirmed by a healthcare provider, and prenatal care should follow their guidance.

Sources

About this page

Publisher:
Kalkoro — Teknolojig Bilişim Teknolojileri A.Ş., İstanbul
Written by:
Kübra Yıldız — Editor (Health, nutrition, pregnancy, education, exam scores)
Reviewed by:
Furkan Tur — Editor

Parameters come from official sources; the last update date is shown at the bottom of the page.

Spotted a wrong result? Use the "Report a bug" button at the top of the page; every report is reviewed.

How we calculate and our accuracy policy →

This tool is for general information only and is not medical diagnosis, treatment or advice. Consult a physician for decisions about your health.

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